1. Field of the Invention
The present invention relates generally to geolocation systems, and particularly to a compact geolocation antenna system.
2. Technical Background
In general, the term geolocation refers to determining the geographic location of some object or thing. For the purpose of this invention, geolocation means determining the location of an object by sensing the radio frequency (RF) signals that the object or system is emitting. Examples of systems or objects that emit RF signals include cell phones, cell phone towers, Wi-Fi hot spots, radars, radio stations and the like. Once the emitted RF energy propagating over the air is detected, the direction (i.e., bearing) of the detected signal can be easily established. The geolocation system may include geographically diverse antennas that obtain the bearing of the RF emitter from various vantage points; and these bearings may be triangulated to find the exact geographical location of the RF emission. A single bearing, or “directional vector,” can also be used in conjunction with other known information or intelligence to locate and identify the RF emitter. For example, the signal strength of a known type of RF emitter (e.g., a certain type of radar) can be used to estimate the range of the RF emitter. As another example, the geolocation system may be equipped with known mapping or intelligence data that can be used with the bearing information to geolocate the RF emitter. These non-limiting examples are not meant to be exhaustive, but rather are meant to give the reader a better understanding of the present invention.
The applications for geolocation range from the exotic to the mundane. In World War II, for example, the British government attempted to use geolocation techniques to detect the radio transmissions of German secret agents operating within the UK. Sixty years later similar techniques are being used in the UK to locate and prosecute “pirate” FM radio broadcasters. Nowadays, one common application for geolocation relates to public safety; emergency radio beacons are used by hikers, skiers, civil aircraft, etc., and are configured to transmit a unique RF signal that can be used by emergency personnel to find the location of the transmitter (and hence the hikers, skiers, aircraft, etc.) in the event of an emergency. In another example application, geolocation may be employed to ascertain the approximate location of a cell telephone handset (and thus the user) by determining which cell tower it is communicating with, determining its bearing to that tower, and estimating the range from the tower as a function of signal strength. Conversely, a user of broadband computing services could employ this technique to find the bearing and range of the nearest Wi-Fi hot spot. As alluded to above, geolocation may be used in a variety of military applications such as determining the location of enemy radars and radio transmissions.
One important component of a geolocation system is the antenna. An antenna with multiple output ports, each sensitive to a particular polarization of the E-Field or H-Field, is called a vector sensor. The conventional “optimal” vector sensor typically includes three dipole antennas and three loop antennas to sense the three Cartesian components of E-Field and of H-Field. It has been thought that such an antenna extracts the maximum available information of a field in a confined region of space. However, contrary to popular opinion, that is not generally the case, as is explained in the following discussion. A loop antenna senses the E-Field directly. The loop voltage is proportional to the differentials in that E-Field and is thus related, indirectly, to the H-Field penetrating the loop. Stated differently, an electrically small loop antenna with constant current is constrained to detect only that combination of differentials that corresponds to the net H-Field penetrating the loop. Thus, the H-Field must be sensed indirectly, i.e., derived from the E-Field differentials, because magnetic conductors do not exist in nature.
In reference to the example diagram depicted in FIG. 1, a loop antenna 1 is disposed in the x, y plane and includes a small rectangular antenna loop 1-1 coupled to port 1-2. In this example a y-polarized plane wave is incident from the −x direction and a reflection wave is incident from the +x direction. If the direct and reflected E-Fields cancel at the origin, the net H-Field at the origin is a maximum, and the loop is sensitive to this H-Field. On the other hand, if an incident and reflected pair of x-polarized plane waves traveling along the y axis are introduced (and also phased to cancel the E-Field at the origin) it also will support a nonzero H-Field at the origin. If the two pairs of waves are phased with respect to each other so that the H-Fields cancel, the loop voltage will be zero. In that case, the loop will not sense either contribution of H-Field. This result is a consequence of the x-polarized components of induced loop current canceling the y-polarized components of induced loop current. In other words, as stated above, an electrically small loop antenna with constant current is constrained to detect only a particular combination of E-Field differentials that correspond to the net incident electromagnetic H-Field. Thus, one drawback associated with a loop antenna is its inability to sense individual components of the incident and reflected fields. Stated differently, because a single loop antenna can be employed to sense only the net H-field penetrating it, it provides only one degree of freedom (DOF) instead of the available two DOFs.
In one approach that was considered, an HF direction finding antenna design was implemented that included crossed loop antennas mounted on a monopole antenna to implement a three-orthogonal-loop-on-a-whip concept. Each loop consisted of an outer larger primary loop and an inner secondary loop. Since the design is based on two loop antennas and one monopole, it can only sense one component of E-Field and two components of H-Field at a point. Thus, the design is inherently limited to three degrees of freedom (DOFs). Moreover, the outer primary loop incorporates capacitive tuning and is an inherently narrow band solution.
In another loop antenna design that was considered, three isolated concentric windings are disposed around a non-conductive cube. Each winding is tuned to a slightly different frequency and attached to a nonaliasing filter and receiver. The purpose of this arrangement is, apparently, to attain a low loss broad operational bandwidth without retuning Nonetheless, this approach does not overcome the drawbacks associated with loop antenna sensors that were identified above, namely, the design is inherently limited to three DOFs.
In yet another conventional approach that was considered, a sensor is implemented that is comprised of three orthogonally oriented loop antenna elements. Each antenna element is designed to sense one Cartesian component of the H-Field. The “twin loop” refers to a “ground symmetrical loop” configuration that is employed to create the symmetry needed for it to function as a balun. As those of ordinary skill in the art appreciate, baluns are often used to suppress feedline currents and provide reasonably stable antenna patterns. This approach includes all of the drawbacks associated with the loop antenna designs described above and, in addition, the design features an impedance mismatch that results in an excessive noise figure (NF) for frequencies below 8 MHz.
In yet another approach that was considered, an antenna array was implemented that included six co-located antennas. The six antennas were comprised of three orthogonal loop antennas and three orthogonal dipoles. This sensor was designed to detect the three Cartesian components of E-Field and three Cartesian components of H-Field at a point. However, this antenna array did not perform especially well in experiments against even one single mode sky wave signal due to the effects of multipath.
What is needed, therefore, is a vector sensor (i.e., a direction finding antenna system) design that provides more than six DOFs to obviate the drawbacks described above while conforming to a form factor that has a relatively small footprint. A vector sensor is needed that has the ability to accurately resolve multiple angles-of-arrival (AoA) signals and capable of distinguishing coherent multipath that might otherwise be indiscernible by conventional vector sensors. Moreover, the vector sensor should only include high impedance ports, rather than the combination of both low and high impedance ports associated with the conventional sensors, to eliminate the impedance matching issues described above. Finally, a vector sensor is needed for broad band applications that may be difficult to achieve with loop antenna designs.